Microstructural Investigation of the Deformation Zone below Nano-Indents in Copper
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1049-AA03-03
Microstructural Investigation of the Deformation Zone below Nano-Indents in Copper Martin Rester, Christian Motz, and Reinhard Pippan Erich Schmid Institute, Austrian Academy of Sciences, Jahnstrasse 12, Leoben, 8700, Austria ABSTRACT The deformation zone below nanoindents in copper single crystals with an < 110 > {111} orientation is investigated. Using a focused ion beam (FIB) system, cross-sections through the center of the indents were fabricated and subsequently analyzed by means of electron backscatter diffraction (EBSD) technique. Additionally, cross-sectional TEM foils were prepared and examined. Due to changes in the crystal orientation around and beneath the indentations, the plastically deformed zone can be visualized and related to the measured hardness values. Furthermore, the hardness data were analyzed using the Nix-Gao model where a linear relationship was found for H² vs. 1/hc, but with different slopes for large and shallow indentations. The measured orientation maps indicate that this behavior is presumably caused by a change in the deformation mechanism. On the basis of possible dislocation arrangements, two models are suggested and compared to the experimental findings. The model presented for large imprints is based on dislocation pile-ups similar to the Hall-Petch effect, while the model for shallow indentations uses far-reaching dislocation loops to accommodate the shape change of the imprint. INTRODUCTION It is well known for many years that the hardness of metals and alloys in the micron and submicron regime is not a constant number. In fact the hardness depends on the size of the indent i.e., with decreasing indentation depth the hardness increases [1-3]. This is called the indentation size effect (ISE). Using the concept of geometrically necessary dislocations (GNDs) and the Taylor rule for the flow stress, Nix and Gao (N-G) proposed a model to explain the ISE [4]. According to this model, a linear correlation between H², the square of the hardness, and 1/hc, the reciprocal indentation depth exists, which is in good agreement with micro-indentation hardness data. In the literature, however, it is reported that nanoindentation hardness data do not follow this linear trend over the whole measurement range [5-7]. Instead, at small indentation depths they start to deviate from the predicted linear curve. In order to verify if this behavior is linked to a change in the deformation structure the plastically deformed volume below different sized nanoindentations is visualized using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results were subsequently used to suggest possible dislocation arrangements in order to explain the indentation process of large and shallow imprints. EXPERIMENT Copper single crystals with an < 110 > {111} orientation were prepared by wet grinding and mechanical polishing. Electropolishing was subsequently performed on the {111} surface in order to remove any deformation layer produced during previous polishing steps
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